(a) Field of the Invention
The present invention relates to a catalyst used in the preparation of methacrylic acid by gas phase catalytic oxidation of methacrolein with molecular oxygen and a method for preparing methacrylic acid which comprises catalytically oxidizing, in a gas phase, methacrolein with molecular oxygen in the presence of the catalyst.
(b) Description of the Prior Art
Up to now, there have been variously proposed catalysts used in preparing methacrylic acid through gas phase catalytic oxidation of methacrolein with molecular oxygen However, these catalysts are incomplete in properties such as catalytic activity, selectivity to methacrylic acid, stability and lifetime. Moreover, if they are prepared in an industrial scale, the quality thereof varies from lot to lot. Therefore, there has been desired to eliminate these drawbacks.
Composite oxide catalysts of Mo V-P-X-Y type (wherein X is at least one element selected from the group consisting of Sb, Cu, Co, Bi and As; and Y is at least one element selected from the group consisting of K, Rb, Cs and Tl), in particular Mo-V-P-Sb-Y type ones, i.e., those having the foregoing compositional formula in which X is Sb have widely been used, in this field, as catalysts for oxidizing methacrolein. In the conventional methods, these catalysts have in general been prepared by mixing aqueous solutions containing catalyst components and oxides, concentrating the mixture to dryness with heating and stirring and then calcining the concentrate. In addition, antimony trioxide has widely been used as a source of Sb as disclosed in, for instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") Nos. Sho 50-101316, Sho 52-68122, Sho 61-114739 and Sho 63 112535. However, it has been known, as disclosed in J.P. KOKAI No. Sho 61-114739, that the catalysts thus prepared by the conventional methods while using commercially available antimony trioxide as an Sb source do not always have satisfied catalytic activity, selectivity to methacrylic acid, stability and lifetime unless the starting antimony trioxide is pulverized into fine particles having an average particle size of not more than 0.2.mu.. However, such pulverization into an average particle size of not more than 0.2.mu. is not practical.
In addition, there have widely been used, in this field, spherical or cylindrical catalysts, but recently there has been proposed the use of ring-like catalysts for preparing methacrylic acid at a high space velocity in high yield. However, such ring-like catalysts are liable to be broken into powder during packaging the catalysts and if they are packaged in a multi-tubular reactor of industrial-scale, the pressure difference in the reactor becomes large due to the pulverization of catalysts into powder and accordingly catalytic performances (activity and selectivity) which would be anticipated in the laboratory scale reaction cannot always be obtained.
In this field, the product (methacrylic acid) has been produced in an amount of several ten thousand tons/year/production apparatus. Therefore, the improvement in yield by only several percentages can provide a significant economical effect. Thus, many attempts have been directed towards the improvement in the initial quality of a catalyst used therein and the development of methods for preparing high quality catalysts in good reproducibility.